Prepping stringers before capping

Hi,

I have tabbed my new stringers in and I was wondering how aggressive to get in prepping them before I put the fiberglass cap on? The edges of the tabbing are not the smoothest but I’m concerned that if I try to sand out every imperfection I’m going to remove a lot of material. Can I use a little fairing compound to smooth things out or is that not a good idea?

 

Bit of both...

Grind it somewhat smooth enough for glass, fill it with thickened resin before cap.

 

Awesome, thanks. Wasn’t looking forward to trying to make all that super smooth, I like the thickened epoxy…

 

Just hand sanding with a 40-60 grit will do. Just remove all the slivers, then fillet all corners with thickened resin about 1/4" radius. Fabric does not conform easily to sharp bends. It is a secondary bond so start with a light CSM before biax. Finish off again with light csm for water proofing or future secondary bond.

 

He's probably using biax or 1708 with epoxy..

 

If my understanding is correct and 1708 has fibres at +/- 45 it might not be the best thing to use for the entirety of the wrapping layers.The stringers really need longitudinal fibres along the top and unidirectional tape is good for this. Using +/- 45 to connect the tape to the rest of the hull is a good thing.If an inboard engine is destined to live on the stringers,it can be useful to include a steel insert where the feet are situated as it can be tapped for the mounting bolts.

 

Very few resellers of glass I have dealt with sell uni. While it may be ideal, most all the DIY boat designers tend to stay away from using it in schedules, probably for same. I am not suggesting you wrong, more that it is interestingly difficult to get stuff.

 

You are correct.

Biax are used because the web of the stiffener are in shear and biax have greater strength at 45 degree than WR, say up to 1.8 X. On the crown where it is in tension/compression, it is weaker than WR (about 45%). Thus the need to reinforce the crown.

To control deflections, there are two basic profile of tophat stiffeners. The low profile or squat type and the tall type. For most low profile use, the standard practice is to overlap the crown at least once to add "meat" to compensate for the loss of longitudinal strength. Or simply insert uni (very strong in longitudinal direction) to compensate for loss of strength using biax. A more complicated approach is to mix WR and biax but it is difficult to calculate because of differing Modulus of Elasticity.

A quick way is just to increase the height of the stiffeners. Use tall tophats. The shear strength may be exceeded but the deflection is controlled. In practice, tall tophats are used for transverses and low profile for stiffeners.

All of these increase stiffness, reducing deflection.

For the engine beds, these are classified as girders, more than a primary stiffener. It is designed to be robust and stiff. Deflection is limited to the amount of misalignment permitted between the shaft and connection to the hull or the amount of misalignment permitted in the flex joints. usually in the range of 2-3 mm.

For girders it is usually made of wood encapsulated with glass. If you want more stiffness without adding bulk, a steel girder/engine bed is bolted to the frame.

 

Is it not true that a uni of weight X can be replaced by a biax (for stringers) for an exchange? For example, is the fiber direction of 10 oz uni at 0 equal to a fiber of biax 140% greater (or so)? So, is the real issue that while uni is best because the theoretical lightest, it can be replaced by a heavier biax?

Keep in mind R, I am not busting your chops, but respect your ability to explain this in detail. I would just be guilty of assuming I'm somewhat correct and hoping to understand the balancing done as I am familiar with designers who never use uni in their schedules..

 

I didn't go back and re-read your other thread but here goes.

Grind the surface so nothing is sticking up, but don't get carried away and try get down to the deepest valley.

Fairing compounds and putty are weaker than the laminate, so don't over use it and create a weak point at the bond line of the two laminates. Using it in the radius is great.

The original laminate, whatever that was, probably wasn't engineered, it was just a type and amount of glass that was a guess and didn't fail. And it may have included a good deal of chop and a couple layers of roving (again, I didn't go back and check this).

So, most likely when using biax and epoxy anything you use that approximates the original thickness will be stronger than when it was built.

If this had originaly been an engineered design with light weight and strength as the goal, my answer would be different.

 

In my situation, I needed 3 layers 1808DBM 45/-45 when the cloth vendor told me just 2 with epoxy, and I needed to align 2 layers to the framing to get similar numbers in Vectorlam as the existing CSM and polyester resin caps. The existing caps held the boat in shape after the plywood 'innards' rotted from water intrusion. Once removed the iron swing keel made the hull bottom sag until I braced it back to nominal.

 

FG- Engineers always use a unit of measure like Force/Area, PSI or N/mm2. Nothing to do with weight.

In a beam formula, the webs (there are 2) takes the shear, the extreme most upper and lower fiber (flange) takes the tension/compression forces.

There are three conditions to be met. The shear on the web, the tensile/compressive stress on the caps, and the amount of deflection permitted.

If the design shows that the webs meets the allowable shear strength but the flange don't, simply add more layer (but only in the flange). Adding more "meat" (by doubling the biax) increases its distance from the neutral axis, reducing the amount of stress the biax need to bear. If that is not enough, Add a layer with a higher strength like a WR. If lacking still, add a Uni which have the highest strength.

Strength and stiffness are two different things.

Now check if if the arrangement/dimensions is adequate in "stiffness". Is it flexing? Is it deflecting more than the allowable? The goal then would be to increase the stiffness (EI) of the structure by any of method described above. A natural way to increase EI is by increasing the height of the stiffener while watching the shear stress on web. Thus the saying, "if not "strong" enough, add more meat, if not "stiff" enough, add more fat".

 

Yikes! I wasn’t paying attention to this thread after I got the answer I was looking for.

yes, I did the tabbing with 1708, four layers with epoxy. Hopefully more than is needed, i t’s very hard to figure what was there, the original construction and the modifications were laid on top of each other in places, it was crazy and it was also done very poorly, lots of air and dry laminates.

I do actually have 12oz unidirectional I can use for the cap. I was coming back to ask about details on that. I have the four staggered edges of the tabbing. Do I do stagger the caps this way?

I saw a response about thickened epoxy/putty bearing weaker between layers. That was kind of what I meant to ask. Is there a general rule about when that becomes an issue? I’m looking at 1/16”-ish thick in spots.

thanks
Jason

 

Jason- Perhaps this will help

 

The image that rxcomposite has posted shows best practice,keep the unidirectional one laminate from the outside as a wrapping over it reduces the chances of it peeling.It doesn't have to be carbon but carbon is the ultimate.Ordinary E glass will still be a lot better than woven rovings or biax for longitudinal strength.From the description it seems that virtually anything will be better than the poor job that was done previously.